Pacing Mode Event Classification with Rate Smoothing and Increased Ventricular Sensing

ABSTRACT

An implantable medical device operates according to a ventricular pacing protocol (VPP) that precludes ventricular pacing in any cardiac cycle where a sensed ventricular event has occurred in the preceding cycle. Improved ventricular sensing, detection and classification is provided.

FIELD OF THE INVENTION

The present invention relates to medical devices and more specificallyto implantable medical devices.

DESCRIPTION OF THE RELATED ART

There are a variety of medical devices that sense data, providediagnostic information, and/or deliver therapy. When such a device isimplantable (in whole or in part), it is referred to as an implantablemedical device (IMD). In the present application, IMD refers to a devicethat senses cardiac events and delivers pacing therapy. Such devices mayor may not also include other functions such as defibrillation therapy(e.g., implantable cardioverter defibrillator (ICD)), other monitoringcapabilities, alternate cardiac therapies, or non-cardiac monitoringand/or therapies. Thus, the term pacemaker may be used interchangeablywith IMD in the present context with the understanding that either termmay refer to a device with capabilities beyond those required of apacemaker alone.

Recently, there has been a recognition that intrinsic conduction andventricular depolarization, even if somewhat prolonged, is preferable toventricular pacing; particularly pacing in or near the right ventricularapex. In general, this preference results from the unnatural propagationof a depolarization wavefront that is generated from such a pacing pulse(as compared to intrinsic depolarization).

Previous pacing modes tend to operate at one extreme or another. Forexample, in a true, single chamber AAI/R device, atrial pacing andsensing is possible, but no ability to provide ventricular pacing (orsensing) exists. On the other hand, DDD/R has historically been thedefault selection for dual chamber devices. The DDD/R mode will operateto maintain AV synchrony; however, the AV delay is necessarily such thatintrinsic conduction is precluded in most cardiac cycles. This resultsin ventricular pacing in a very high percentage of cardiac cycles.

The present assignee has developed new modes that promote intrinsicconduction and are referred to herein generally as ventricular pacingprotocols (VPP). One such VPP is Managed Ventricular Pacing™ (or MVP™)which is commercially available. A variety of VPP embodiments havepreviously been described, for example, as in U.S. Pat. No. 6,772,005,issued Aug. 3, 2004, to Casavant et al., (Atty. Docket No. P9039.00);U.S. application Ser. No. 10/246,816, filed Sep. 17, 2002, (Atty. DocketNo. P9039.02); U.S. application Ser. No. 10/755,454, filed Jan. 12,2004, (Atty. Docket No. P9039.06); U.S. application Ser. No. 10/850,666,filed May 21, 2004, (Atty. Docket No. P21161.00); U.S. application Ser.No. 11/115,605, filed Apr. 27, 2005, (Atty. Docket No. P21164.00); U.S.application Ser. No. 11/096,436, filed Mar. 31, 2005, (Atty. Docket No.P21166.00); U.S. application Ser. No. 10/814,692, filed Mar. 31, 2004,(Atty. Docket No. P20243.00); U.S. application Ser. No. 11/364,290,filed Feb. 28, 2006, (Atty. Docket No. P21506.00); U.S. application Ser.No. 10/971,686, filed Oct. 25, 2004, (Atty. Docket No. P20851.00), U.S.application Ser. No. 11/424,410, filed Jun. 15, 2006, (Atty. Docket No.P25415.00), U.S. application Ser. No. 11/424,383, filed Jun. 15, 2006,(Atty. Docket No. P26089.00), U.S. application Ser. No. 11/424,395,filed Jun. 15, 2006, (Atty. Docket No. P26091.00), and U.S. applicationSer. No. 11/424,405, filed Jun. 15, 2006, (Atty. Docket No. P23959.00),which are herein incorporated by reference in their entirety. Otherrelated applications include U.S. application. Ser. No. 11,258,523,filed Oct. 25, 2005, (Atty. Docket No. P20923.00) and U.S. applicationSer. No. 11/257,643, filed Oct. 25, 2005, (Atty. Docket No. P21974.00).

As a generalized explanation, a VPP operates in an atrial based pacingmode to promote intrinsic conduction. Ventricular events are sensed andas long as a ventricular event is sensed in a given cardiac cycle (e.g.,an A-A interval) the device continues to operate in the atrial basedpacing mode. This allows for ventricular sensing during the entire A-Ainterval. Conversely, if there is no ventricular event, the deviceprovides a ventricular backup pace in the subsequent cycle, timed fromthe atrial event (paced or sensed) that initiates this subsequentcardiac cycle. Thus, in a VPP it is possible to have an entire cardiaccycle devoid of ventricular activity while ultimately maintaining AVsynchrony. There are, of course, many variations and embodimentsprovided that are not described herein for the sake of brevity. Itshould be appreciated that operation in an atrial based pacing modeincludes mode switching a device into such a mode (e.g. AAI/R, ADI/R)and into a mode that provides ventricular pacing (e.g., DDI/R, DDD/R,VVI/R, etc.) as necessary and potentially on a beat by beat basis oralternatively, operation in a complex mode that includes morecomprehensive behavior (e.g., FIDDI) without necessitating modeswitching to achieve the functionality described.

One benefit of a VPP is that the protocol may be initiated with patientsregardless of the status of their AV conduction. Those having intact orpartially intact conduction will benefit in that conduction is promotedand ventricular pacing is reduced or eliminated. For those patients withheart block, the VPP will quickly move to provide ventricular pacing andperiodically check to determine if conduction has returned. Both ininitially recognizing the need to pace and performing the conductionchecks, the methodology employed is transparent to the patient.

As previously indicated physicians implanting a dual chamber deviceoften utilize nominal settings and program the device to DDD/R due toits simplicity. The VPP allows for the same type of comprehensivereliability across patient profiles and without the need to programnumerous parameters upon implant. The VPPs are preferable in that thatthey reduce or minimize ventricular pacing when intact conduction ispresent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an implantable medical deviceconsistent with the teachings of the present invention.

FIGS. 2-4 are timing diagrams.

FIG. 5 is a graph illustrating ventricular pace energy dissipation.

FIG. 6 is a timing diagram.

FIGS. 7-9 are flowcharts describing various processes consistent withthe present invention.

FIG. 10 is a block diagram illustrating components of an implantablemedical device consistent with the teachings of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an implantable medical device (IMD) 10having pacing capabilities. While not illustrated, IMD 10 may alsoinclude a variety of other monitoring, diagnostic and therapeuticfunctions. Further, FIG. 1 is not meant to comprehensively illustrateall components of an implantable pacemaker.

The IMD 10 includes a housing 12 that contains a microprocessor 14,memory 16, a power supply (e.g., battery) 18, a communication module 20that facilitates telemetry to an external device and a pulse generator22 for generating pacing pulses. A rate response module 25 is providedto optionally obtain sensory input and control a pacing rate based uponperceived physiological need. A sensor interface 30 is provided tocollect data from one or more sensors/electrodes, one or more of whichmay be disposed on leads 32, 34. The pacing stimuli generated by thepulse generator 22 are delivered via the leads 32, 34. Also illustratedin FIG. 1 is a VPP module 24. It should be appreciated that thesefunctions may be algorithms stored in the memory 16 or incorporated intoother hardware, software, or firmware.

In operation, the IMD 10 senses cardiac events and provides anappropriate response. Most typically, cardiac events are sensed viaelectrodes on the leads 32, 34. The input is passed through an atrialsense amplifier 36 or a ventricular sense amplifier 38, and the signalfrom the amplifier(s) is then processed. These processed signals areindicative of specific activities within the heart, typicallyrepresented as an electrogram (EGM) when generated from device data oran electrocardiogram (ECG) when based upon surface collected data. Analternative sensor 35 is illustrated as being in communication with theIMD 10 and may represent another implanted sensor in direct or indirectcommunication with the IMD or an external sensor such as ECG electrodesthat may or may not be in communication with the IMD 10. As is wellknown, the cardiac cycle includes an atrial depolarization representedelectrically by a P wave, ventricular depolarization represented by theQRS complex, and repolarization represented by a T wave. While sensingalgorithms can be relatively complex, in general a sensed P waveindicates intrinsic atrial depolarization while a sensed R waveindicates intrinsic ventricular depolarization. For a given pacing mode,if a P wave or R wave is not sensed within a predetermined time frame,then the IMD 10 provides atrial or ventricular pacing with appropriatetiming, if supported by that mode. There are numerous variations to thisgeneralization such as overdrive pacing or various tachycardia pacingtherapies. The main point herein is that the IMD 10 senses data andresponds in some fashion to that data according to the parameters of aselected mode.

As discussed, the present invention relates to an IMD 10 thatselectively operates according to a VPP, such as for example, the MVP™mode. There are numerous variations among the VPPs and for the sake ofclarity not every variation will be separately described.

FIG. 2 is a generalized timing diagram illustrating certain cardiacevents as sensed on an atrial (A) channel, a ventricular (V) channel,and over an “other” channel. It should be appreciated that this diagramand those like it are not meant to accurately reflect waveforms orprovide accurate temporal proportions or relationships. The “other”channel may include surface EKG, far field sensing, separate leadsensing, can electrode(s) or any sensing technique other than thelead/electrode for the particular atrial or ventricular chamber when andwhere a relevant event is occurring. At time T1, an atrial pace (AP) isdelivered and is illustrated as a marker channel spike on the atrialchannel (and could alternatively have been illustrated as a waveform).The resulting depolarization is illustrated, schematically, as awaveform on the “other” channel. Notably, no activity from the AP ispresent on the ventricular channel. This results from a post-atrialventricular blanking period (PAVB). During this time, the ventricularsense amplifier 38 is disconnected (literally or effectively) from theventricular lead 34 as the atrial pacing pulse would generate anelectric field that would at the very least be sensed by the ventricularlead 34 and potentially “overwhelm” the sense amplifier 38 due to theamplitude of the signal. Absent the PAVB and other controls, twonegative results could occur. The first is that the large signalproduces residual effects that prevent accurate sensing with theventricular lead, even after the atrial waveform has apparentlydissipated. The second is that the atrial event is interpreted as aventricular event on the ventricular channel. This is referred to ascrosstalk. In other words, an atrial event is sensed (far field) on theventricular lead 34 and considered as a ventricular event.

By disconnecting the sense amplifier 38 when an atrial pace isdelivered, these effects are prevented from occurring. On the otherhand, if a true ventricular event does occur during this time, it cannotbe sensed on the ventricular channel. In addition to the PAVB, manypacing modes include a crosstalk window. This window extends beyond thePAVB; however, the sense amplifier 38 is connected. Thus, events can besensed on the ventricular channel (including crosstalk). In general,events sensed during the crosstalk window are assumed to be crosstalkand are treated as such. The distinction is that during the PAVBventricular sensing is precluded; during the crosstalk window sensing ispermitted and the window simply defines how sensed data may beclassified. It should be appreciated that these events are not drawn toscale nor are they proportionally accurate. In a typical device, thePAVB may be on the order of 30 ms and a crosstalk window may be on theorder of 50-100 ms. Thus, a total interval would be 80 to 130 ms withthe PAVB representing less that half of this duration. Thus, theproportions shown are provided for ease of illustration and tofacilitate description and do not represent accurate temporalproportions.

Following the expiration of the crosstalk window, events sensed on theventricular channel are generally classified as conducted events orother true ventricular events. Such an event is illustrated as aventricular sensed event (VS) at time T2. At time T3, another AP isdelivered and the PAVB (1) and crosstalk window (2) run. Referring tothe “other” channel, three “events” are illustrated. The first event E1is the waveform resulting from the atrial pace AP; just as it occurredat time T1. The second event E2 is a premature ventricular contraction(PVC). As this occurs during the PAVB, it will not be sensed on theventricular channel. The third event E3 is also a PVC, but this PVCoccurs during the crosstalk window 2. Thus, it will be sensed on theventricular channel; however, according to many pacing modes, this PVCis “ignored” as crosstalk. Thus, regardless of whether either PVC E2 orE3 occurred, a ventricular pacing pulse VP is delivered at time T4(according to the current mode selection, e.g., DDD/R). It should beappreciated that having two PVCs as illustrated is not likely and theyare illustrated as such simply to show a PVC in each window. Withcontinued operation in this mode, the next interval begins with anatrial pace AP at time T5. This might be typical operation in a modesuch as DDD/R.

FIG. 3A is a timing diagram illustrating the relative timing that occurswhen a VPP, such as for example, the MVP™ mode is utilized. In previousVPPs, the PAVB and crosstalk window exist, substantially as describedabove. As indicated, in a VPP mode, ventricular pacing is generally notavailable in a given cardiac cycle (A-A) interval, where a ventricularevent occurred in the preceding cardiac cycle. Thus, at time T2, a VSoccurs. Therefore, ventricular pacing will not be provided in the A-Ainterval defined by T3 and T4. As illustrated, no ventricular event issensed in this interval. As such, a ventricular pace (VP) is deliveredin the next cardiac cycle at time T5.

FIG. 3B continues with the same VPP. At time T3, an AP is delivered andjust as illustrated in FIG. 2, three events are illustrated on the“other” channel. E2 and E3 are PVCs (again unlikely to both occur; showntogether for illustrative purposes). The PVC at E2 cannot be sensed onthe ventricular channel, as the sense amplifier 38 has beendisconnected. The PVC at E3 can be sensed; however, since it is in thecrosstalk window 2, it is ignored (i.e., believed to be atrial crosstalkrather than a PVC). In either case, no ventricular event is “sensed”(more accurately, no sensed ventricular event is classified as aventricular event) and according to the VPP a ventricular pace VP isdelivered in the next cycle at time T5. While various embodiments of theVPPs exist, many include a provision that any ventricular event,including a PVC, will “count” as a ventricular event and thereforeventricular pacing will not be provided in the subsequent A-A interval.Because the PVCs occurred during the PAVB and/or the crosstalk window,what may be considered an “unnecessary” ventricular pacing pulse isdelivered at T5.

A similar scenario is illustrated in FIG. 3C. Here, the PVC E2 isillustrated. Due to the nature of the PAVB, this PVC cannot be sensedand will result in the VP at time T6. As indicated, this results inaction that is contrary to normal VPP operation if such an event weresensed and classified as a PVC. In this example, a ventricular eventdoes occur very early in the A-A interval between T3 and T5; however,this event (PVC E2) is within the PAVB window and therefore could not bedetected. The ventricular pace VP is delivered rather early in the A-Ainterval initiated by the AP at time T5. This is potentiallysignificant, in that a relatively short V-V interval (T2-T4) occurs andis followed by a relatively long V-V interval (T4-T6) that is terminatedby ventricular pace. In a minor subset of patients, there is speculationthat this type of scenario could be proarrhythmic and lead to pacemakerinduced tachycardia (PIT). The validity of this concept is not presentlyknown. What further complicates the matter is that the “other” channeldata is often unavailable. Thus, when reviewing collected data frompatients having a ventricular arrhythmia following a VP, the presence ofthe PVC in the PAVB is often unknown. As such, one might correlate thenormal operation of the VPP to causation of the arrhythmia, when in factthe PVC is an unrecognized intervening factor.

FIG. 3D illustrates PVC E3 occurring during the crosstalk window 2. Inother embodiments, a sensed event occurring in this window is treated ascrosstalk and effectively ignored. In alternate embodiments of VPPs,events occurring during the crosstalk window are not automaticallydismissed. Such embodiments are described in Atty. Docket No. P21161.00,entitled “Ventricular Event Filtering for an Implantable MedicalDevice”, Ser. No. 10/850,666, filed May 21, 2004, which is hereinincorporated by reference in its entirety. In one such embodiment, thefirst (or first few) occurrence(s) of an event in the crosstalk windowis treated as a PVC. If the event were truly crosstalk, then it wouldlikely be repetitive and occur in subsequent cardiac cycles. If that isthe case, these repetitive events are treated as crosstalk in thosesubsequent occurrences. When occurring rather infrequently, they aremore likely a PVC and are treated as such. Thus, as illustrated in FIG.3D, using such an embodiment, the PVC E3 is sensed as a VS at time T6.This “qualifies” as a ventricular event and in the subsequent cycle,ventricular pacing is not provided and a conducted ventricular event VSoccurs at time T6. Thus, the above referenced application addresses PVCsoccurring in the crosstalk window and may be included in variousembodiments of the present invention. In the alternative, if ventricularsensed events occurring during the crosstalk window are not treated asPVCs, there presence will at least be noted upon subsequent review ofdata if any anomalies follow a ventricular pace delivered in thesubsequent cardiac cycle.

FIG. 4 is a timing diagram illustrating one aspect of the presentinvention. The PAVB, in these embodiments, is considerably shortened.Typically, a PAVB with traditional modes (e.g., DDD/R) is on the orderof 30 ms. The truncated PAVB is significantly shorter. In oneembodiment, the truncated PAVB is 8.5 ms, though this is non-limiting.The truncated PAVB need only be sufficiently long so that theventricular sense amplifier is disconnected when the true effects of theatrial pacing pulse will occur. Thus, in an IMD 10 capable of multiplemodes, two or more PAVB intervals are retained in memory and used asdescribed herein.

This approach is counter to the conventional wisdom and crosstalkcertainly could occur beyond the truncated PAVB. The truncated PAVB issuccessful when used with a VPP because the present inventors haverecognized that it is ventricular pacing that typically createscrosstalk (or what has been perceived as or attributed to crosstalk)beyond the truncated window. FIG. 5 is a graph of energy versus time atan interface between an electrode and tissue. At time T1, a ventricularpacing pulse is delivered (solid line) and an intrinsic ventriculardepolarization occurs (dashed line—offset for visibility); these eventsare not drawn to scale. The magnitude of the pacing pulse is vastlygreater than the intrinsic depolarization; particularly at thetissue/electrode interface. The pacing pulse tends to createpolarization about the electrode that remains for some time after thepulse. At time T3, the ventricular sense amplifier is disconnected for aPAVB and reconnected at time T4. The polarization at the interfacegenerates a signal that is perceived by the sense amplifier 38. Whendisconnected, the sensed amplifier 38 “holds” at this value. Thus, whenreconnected, the voltage sensed is lower than the held value (due to thedecay of the polarization artifact); hence there is an energy deltabetween sensed values at T3 and T4. This is perceived as a ventricularevent at T4 when in fact it is “crosstalk.” Of course, this is not trulycrosstalk (from an atrial event) in that is an artifact occurring on theventricular lead, but is something that should be ignored just the same.FIG. 5 also illustrates that this effect does not occur from conductedevents as polarization proximate the electrode is not a factor.

Returning to FIG. 4, the truncated PAVB may be used when ventricularpacing has not recently been provided. At time T5 an atrial pulse occursand the truncated PAVB runs. At time T6 a ventricular pacing pulse VP isdelivered. In the next cycle, a “normal” (i.e., long) PAVB is utilizedbecause the polarization from the previous VP may generate errantsignals.

FIG. 6 illustrates use of the truncated PAVB with a VPP. At T1, an AP isdelivered and a truncated PAVB is initiated. A ventricular event issensed at T2, precluding ventricular pacing in the next cycle, whichbegins at T3 and has a truncated PAVB. As illustrated, no ventricularevent is sensed and at T4, the next AP is delivered and anothertruncated PAVB begins. A ventricular backup pace VP is delivered at T5.In the next cycle, a longer PAVB is used and begins at T6. In the nextthree cycles illustrated, conducted ventricular events are sensed.Typically, a normal or “long” PAVB is only necessary in the cyclefollowing the ventricular pacing pulse. The polarization effects tend todecay over a known time period. While not limiting, this has beenobserved to be about 800 ms. Thus, in one embodiment, the microprocessorof the IMD 10 calculates the time since delivery of a ventricular pacingpulse. Until this predetermined interval expires, any atrial event willinitiate a longer PAVB. As such, at higher heart rates it is possible tohave multiple cycles that utilize the longer PAVB related to a singleventricular pace. In other embodiments, the interval may be set to avalue between about 400-2000 ms. Alternatively, the value may equate toa number of cardiac cycles which may be fixed or vary with heart rate.

As illustrated in FIG. 6, an 800 ms timer is initiated at time T5. Asthis timer does end prior to T8, the PAVB started at T8 is a truncatedPAVB. The PAVB illustrated at time T8 illustrates another concept. Thatis, in addition to running a truncated PAVB, a crosstalk window may runand may terminate after the same duration as previously utilized. Thatis, the crosstalk window expired at some fixed interval following theatrial pace; with a longer PAVB, this resulted in a shorter crosstalkwindow. With a truncated PAVB, the crosstalk window may be lengthened soas to terminate at the same point in time relative to the atrial pace.Events are sensed during this window and may be classified as crosstalkor as a PVC depending upon the protocol parameters. The effect of thetruncated PAVB, with or without a crosstalk window is that accurateventricular sensing occurs. Thus, PVCs are not likely to be undersensedand overlooked as described in previous examples.

FIG. 7 is a flowchart that describes the use of a truncated PAVB with aVPP. Initially, the IMD 10 begins operation (100) according to theparameters of the VPP. For illustrative purposes, we assume that therehas not been ventricular pacing for some time. The PAVB is set (110) tothe truncated or short value, which in one embodiment is 8.5 ms. The IMD10 then monitors to determine if a ventricular pace (120) is delivered.Assuming there is no ventricular pacing, the PAVB is maintained at theshort value (110). Thus, any event occurring after this short PAVB maybe sensed on the ventricular channel. Alternatively, if a ventricularpace is (120) delivered, the PAVB interval is set to the long value orwhat is currently the “normal” PAVB, which may be approximately 30 ms inone embodiment. Thus, at the next atrial event, the PAVB will be at thelonger value.

A decay timer is initiated (140) to determine when to revert to theshort PAVB. As indicated, this may be a specific time interval (e.g.,800 ms) or may a particular number of cardiac cycles (e.g., 1, 2, 3, 4).Alternatively, polarization at the ventricular lead may be measuredand/or a patient specific decay timer may be set accordingly.Subsequently, the IMD 10 continues to monitor for ventricular pacing(150). If ventricular pacing (150) occurs while the decay timer isrunning (or prior to the lapse of the predetermined number of cardiaccycles), the PAVB is reset to the long value (130) and the decay timeris reset (140). If no ventricular pace is delivered (150), the IMD 10continues to monitor until the decay timer expires (160). Uponexpiration, the PAVB is reset to the short value (110) and the processrepeats.

In this manner, true ventricular blanking only occurs for a relativelyshort time period following atrial events, thus providing an increasedability to sense events, including PVCs on the ventricular channel. Whenventricular pacing is provided, this PAVB is lengthened to avoidpotential effects generated by polarization.

FIG. 8 is a flowchart illustrating VPP operation with a short PAVB and acrosstalk window event classification. The IMD 10 begins (200) operationin the VPP and the PAVB is set to the short interval (205). Events aresensed (210) on the ventricular channel. If no event is sensed 215, theIMD 10 determines if the A-A interval has expired (220), if not, the IMD10 continues to monitor the ventricular channel (210). If the A-Ainterval has expired (220), then ventricular pacing will be permitted inthe next A-A interval (which at this point in time is now the currentA-A interval); in other words, ventricular pacing is permitted in an A-Ainterval following an A-A interval devoid of ventricular events.Assuming the ventricular pace is delivered, the PAVB is set to the longvalue and the process (235) follows that described in FIG. 7 beginningwith step 140. If the ventricular pace is inhibited, then the processmoves from step 225 to step 215 with a sensed ventricular event.

If a ventricular event is sensed at step 215, the process moves to step240. It should be appreciated the process may move to step 240 followingany sensed ventricular event whether the PAVB is set to short or long.In either case, the IMD 10 determines if the sensed ventricular eventoccurred (240) during the programmed crosstalk window. If not, normaloperation according to the VPP occurs (255). If the ventricular eventdid occur during the crosstalk window (240), the IMD 10 determines ifthis is the first such occurrence (245). If yes, then this event(occurring during the crosstalk window) is treated as a PVC (260) andthe VPP proceeds accordingly (255). In one such embodiment, the VPPconsiders the PVC as a ventricular event that precludes ventricularpacing in the next cardiac cycle. If not the first occurrence (245), theventricular sense is classified as “crosstalk” (not a ventricular event)and the VPP proceeds accordingly (255). It should be appreciated thatthe determination of whether this is a first occurrence (245) isrelative. As explained, true crosstalk is usually repetitive; thus eachconsecutive cycle will have the same type of event. Thus, the firstoccurrence is after some predetermined number of cycles devoid ofsimilarly timed events. Furthermore, some embodiments may require morethan 2 consecutive events to occur before classifying them as crosstalk.In such a case, the first, second, and third such events, for example,may be classified as PVC's (245, 260).

As previously indicated, certain events or patterns might beproarrhythmic in certain patients and the following operation isprovided to determine or at least correlate or suggest what may be apacemaker induced tachycardia (PIT) with VPP operation, whereappropriate. If arrhythmia (ventricular tachycardia in this example,thus VT) occurs, the events preceding the VT are analyzed. There areseveral possible scenarios. In the first scenario, no ventricular pacingwas provided in the cycle immediately prior to (or alternatively, in atime frame where such pacing is established to be relevant to) theearliest onset of the VT. In this case, the VT is not PIT and nocorrelation is drawn with the VPP.

In the second scenario, a ventricular pace was delivered in the cardiaccycle immediately preceding the onset of the VT. In addition, a PVC orconsecutive PVCs occurred, where either the PVC or if consecutive, firstPVC was not treated as a sensed ventricular event in the cycle beforethe ventricular pace. If there is a correlation to be drawn with theVPP, this is the most likely scenario. As explained above, thissequencing results in a short V-V interval, followed by a long V-Vinterval that is terminated with a ventricular pace. With the variousoptions provided herein (shortened PAVB, feed forward classification,rate smoothing, etc.), the likelihood of blanking a PVC is reduced and asensed PVC would be treated as a sensed ventricular event and thereforenot result in a ventricular pace in the next cycle and/or avoid theshort/long patterning. However, some embodiments would not “count” asensed PVC and in those cases, this pattern could be associated with aPIT. As the likelihood of these events actually being related is rare,it is also quite likely just coincidental. Thus, in one embodiment, thispattern must repeat a predetermined number of times before declaring aPIT and taking an action such as disabling the VPP (e.g., switching toDDD/R). This assumes that the patient has an ICD; if the patient onlyhas a pacemaker (IPG), then one instance may be sufficient to disablethe VPP as there is generally no therapy provided to address the VT,regardless of its cause. These options regarding the number ofoccurrences before altering the VPP are ultimately left to theprogramming physician to determine the best therapeutic options for thepatient.

In the context of the present invention, the most likely scenario thatmight be VPP associated PIT is therefore the least likely to occur whenthe various aspects of the invention are utilized together in variouscombinations. That is, PVCs are more likely to be sensed with thepresent invention and when PVCs are sensed they are treated asventricular events for purposes of precluding ventricular pacing in thenext cycle. Alternatively, or in addition, rate smoothing can beutilized to avoid the short/long patterning described. Of course, asthese features are programmable if this VPP associated PIT patternoccurs and, for example, shortened PAVBs were disabled, then theoccurrence of this event may be a trigger to enable such a featureeither under physician control or automatically.

In the third scenario, a ventricular pace also immediately precedes theonset of the VT; however, there is no sensed PVC that occurs in thecycle before the ventricular pace. This is the most difficult scenarioto assess. In one case, there simply is no PVC that occurred in therelevant cardiac cycle. The most likely example is a single,non-conducted atrial event outside of the refractory period, followed bya ventricular backup pace in the subsequent cycle. This pattern ispermitted under the VPP. There is a remote and speculative chance thateven in the absence of a PVC, a long pause due to the above describednon-conducted atrial event without ventricular activity followed by acycle with a short AV interval terminated with a ventricular pace may berelated to PIT in isolated patient cases. Because the present data doesnot support a probability of correlation, a single occurrence shouldpreferably be weighted away from determining that a PIT occurred as thisis more likely purely coincidental; however, this may optionally lead tosuspension or disablement of the VPP (indicated as a dashed line in FIG.9). Again, if the patient does not have an ICD, any VT should be takenseriously and addressed accordingly.

In a fourth scenario, there is also a ventricular pace immediatelypreceding the onset of the VT and a PVC did occur; however, it was notsensed by the IMD 10. This would be referred to a true undersensing of aPVC, related most likely, to a hardware, software, or device issue. Forexample, a problem may develop with the IMD 10, the lead 34, a sensingelectrode, or some other component(s), that results in undersensing ofthe PVC. This would most likely result in undersensing of allventricular activity and would be addressed accordingly; however, thiscould effectively appear as the second scenario, under the rightconditions. It should be appreciated that this fourth scenario is notrooted in any issued with the VPP, and would affect the IMD 10regardless of the mode.

The IMD 10 will take certain actions if the fourth scenario occurs. Ifavailable, alternative sensor data is evaluated to try to determine if aPVC occurred that was not sensed on the primary sensing mechanism, e.g.,on the ventricular lead. If a PVC is found in this manner, this movesthe event to the second scenario. Assuming no PVC is identified (or nosuch alternative sensing exists), then the IMD 10 determines that the VTwas preceded by a VP that was not preceded (in relevant time) by a PVCand/or there is a recognition of an undersensing problem. The actiontaken, if any will be programmed by the physician and any issue leadingto undersensing, once identified, would be addressed in the knownmanner.

FIG. 9 is a flowchart illustrating a process for selectively altering ordisabling a VPP in certain circumstances, in accordance with the abovedescribed three scenarios. Once again, the IMD 10 is initially operatingaccording to a selected VPP (300). This may include but does not requiresetting a PAVB length based upon ventricular pacing and/or classifyingevents in a crosstalk window. As operation continues in the VPP, the IMD10 monitors (310) for ventricular arrhythmias, particularly ventriculartachycardia or ventricular fibrillation (a similar process may beemployed as described for atrial arrhythmias). If such an arrhythmiaoccurs, the IMD 10 provides (315) the appropriate therapy which mayinclude operation in a mode other than the VPP for some period of time.The present methodology assumes successful termination of theventricular arrhythmia and the ability to return to operation in theVPP. If the IMD 10 does not have a therapy option for the ventriculararrhythmia, the process presumes that the ventricular arrhythmia selfterminates or is addressed by another medical treatment. A determinationis made as to whether a ventricular pace (delivered according to theVPP) preceded (320) the initiation of the ventricular arrhythmia. Thismay include immediately preceding the event or up to a set number ofcardiac cycles (e.g., 1, 2, 3, 4 or more) where the events are likely tobe related. In one embodiment, the ventricular pace must occur in thecardiac cycle immediately prior to the onset of the arrhythmia to berelevant to the current decision process. The longer the time periodbetween a ventricular pace and a ventricular arrhythmia, the less likelythey are related; however, the time frame may be programmed in someembodiments.

If the ventricular arrhythmia was not preceded by a ventricular pace(320) in the relevant time period, then operation of the VPP is notlikely a cause or a factor related to that event. Thus, operation in theVPP is permitted (300). This is scenario one, as described above.

It should be appreciated that one or more particularly long pauses,terminated by an intrinsic ventricular depolarization might bypro-arrhythmic in a very small subset of patients having long QTintervals. Thus, while not separately illustrated, ventriculararrhythmia proceeded by relatively long pauses where the patient isknown to have or is sensed to have long QT intervals may be treated in amanner similar to that described below for instances where a ventricularpace does precede arrhythmia, even though no ventricular pace occurs. Inother words, if long pauses and long QT intervals are demonstrablyrelated to ventricular arrhythmia, then various options are availablesuch as rate smoothing (as described herein) or switching to a non-VPPmode to avoid these pauses.

Returning to FIG. 9, if a ventricular pace did precede the onset ofventricular arrhythmia (320), then the IMD 10 determines if there was aPVC prior to the ventricular pace. In one embodiment, the ventricularpace must occur immediately prior to the onset of the ventriculararrhythmia and the PVC must occur in the cardiac cycle immediately priorto that cycle when the VP was delivered. If a PVC occurred (325), thenit is possible that this event and the ventricular pace could lead toPIT and as such, the VPP may be disabled (340). This is scenario two, asdescribed above. This may include operation in true AAI/R (345), VVI/R,a combined AAI/R VVI/R mode where the ventricular rate is very low(e.g., 30-40 bpm), or DDD/R (350). In other words, operation in the VPPis terminated or suspended temporarily or permanently; this includessuspending periodic conduction checks that would normally occur in,e.g., DDD/R. This change may occur automatically or may be made manuallyby a caregiver programming the IMD 10 based upon data collected by thedevice. In general, a change out of the VPP is less desirable, but if itmust be made changing to a comprehensive mode such as DDD/R ispreferred. AAI/R would not provide ventricular pacing even if required,VVI/R does not provide atrial pacing, and the combined AAI/R VVI/R doesnot maintain AV synchrony.

As indicated, the vast majority of patients not only tolerates butbenefit greatly from utilization of the VPP. It is not believed likelythat this algorithm is likely to facilitate ventricular arrhythmia.Thus, because of the low likelihood of correlation, more than one suchevent may be required prior to disabling backup pacing (340) (i.e.,VPP). That is, two or more occurrences of ventricular arrhythmiaproceeded by ventricular pacing (within a VPP) with a PVC and withoutother discernable causes may be required before disabling the VPP, inone embodiment. Again, this assumes that the patient has an ICD. If thepatient has only a pacemaker, one such occurrence may be sufficient todisable the VPP (340) out of an abundance of caution, even though acorrelation to the VPP remains unlikely. Ultimately, these areprogrammable features selectable by the caregiver.

Returning to step 325, the IMD 10 may not be able to determine if asensed signal is a PVC. Thus, the relevant cardiac cycles are evaluatedto determine if there is a discernable pattern that would cause theundetermined event. If such a pattern exists, then the sensed data islikely crosstalk and not a PVC; thus, the process proceeds to step 335.If no such pattern exists, then the event may be classified as a PVC andif appropriate, the process proceeds to step 340. Whether there is or isnot a pattern, additional information may be obtained (335) whileoperation in the VPP continues. The additional information may bemonitoring subsequent cardiac cycles to identify a pattern, if present.That is, due to the low likelihood of an association between operationin the VPP and PIT, questionable events may be further evaluated for aperiod of time before taking an action.

In an alternative embodiment, additional steps are taken to identifywhether or not a PVC is present (325) in a relevant cardiac cycle. Asexplained, the PAVB may be truncated to allow direct sensing on theventricular channel for a greater period of time. Even so, there isstill a brief period of time where such sensing is disabled.Furthermore, it is possible that a “long” PAVB is running due to anearlier ventricular pacing pulse. This would be most likely to occur ata higher pacing rate. Of course, a given embodiment may not have theshort PAVB capability or have that capability programmed off (in otherwords, a clinician may choose to disable the short PAVB functionality).Finally, for any number of possible reasons a PVC may occur that issimply not sensed by the ventricular lead whether due to a problem withthat lead; an anomalous condition; extraneous noise; or some otherfactor.

When appropriate, a concerted effort may be made to identify such anevent, beyond relying upon direct sensing on the most likelylead/electrode (e.g., ventricular pace/sense electrode). Thus,alternative sensor 35 and/or atrial lead 32 may be utilized to attemptto identify the presence of a PVC (or any undersensed ventricularevent). This action may be taken in every cycle; taken in every cyclebut only after an initial suspect arrhythmia has occurred; or the datamay be stored (and not immediately processed) and only processed whenthere is a lack of ventricular activity indicated by the primary sensinglead (ventricular lead 34 in this example).

The atrial lead 32 may be used to detect the ventricular events via farfield sensing. The alternative sensor 35 may include otherlead/electrode combinations (e.g., a can electrode) or other sensingdevices that are either implanted or external. For example, in a suspectpatient a Holter monitor or implantable loop recorder may be provided tosense and collect such data either for subsequent review or may be incommunication with the IMD 10. In summary, multiple sensing techniquesare available to determine whether or not a PVC occurred (325).

FIG. 9 primarily addresses scenarios one and two as previouslydescribed. It should be appreciated that the third scenario (dashedline) would be similarly illustrated and rather than resuming normal VPPoperation if no PVC is detected, the VPP may be disabled (340) after a“yes” in step 320. Again, there is a greater chance that such events arecoincidental rather than related; however, the option to disable orsuspend the VPP is provided. Should more than one such event sequenceoccur, the probability of correlation increases.

It should be appreciated that other relevant events may have lead to orcontributed to the ventricular arrhythmia. Some of these events may besensed by the IMD while many may not. Thus, in some embodiments, anydecision to terminate or suspend operation of the VPP is made manuallyby a clinician. For example, a recent medication adjustment or misseddose, particularly strenuous exercise, high stress levels, otherillnesses or a plethora of other factors may have caused or contributedto the arrhythmia. The clinician may be able to identify these factorswhen consulting with the patient whereas the IMD would not have theability to obtain such information. As such, ventricular pacing may becoincidental with the arrhythmia and not a cause.

As described in more detail in commonly assigned, previously referencedcopending applications P25415.00, P26089.00, P26091.00 and P23959.00,which are herein incorporated by reference in their entireties, varioussmoothing functions are provided in combination with VPP operation. Thesmoothing function may be specifically utilized in the event of a PVC ormay be utilized when patient specific information indicates that aventricular sense is unlikely in a given cardiac cycle. While referenceshould be made to the incorporated documents, a summary is providedherein for illustrative purpose and should not in any way be interpretedto limit the copending applications.

VPP smoothing, as indicated, has two generalized variations. The firstis used when a PVC occurs either very early in an A-A interval or aftera properly conducted ventricular event. With an early PVC, V-V intervals(including the PVC as a V event) will tend to vary, even with a fixedatrial rate. Thus, there may be a normal V-V, followed by a relativelyshort V-V, followed by a relatively long V-V. In some cases, the lastV-V may be quite long and may be terminated by a ventricular pacingpulse. As previously described, a VPP does not control ventriculartiming and only provides ventricular pacing in a cardiac cycle where theprevious cardiac cycle was devoid of ventricular activity. Thus, tosmooth these V-V variations, the VPP accelerates atrial timing; that is,an atrial pace is delivered early (with respect the current atrialrate). This shortens the A-A interval that includes the PVC and assumingintrinsic conduction in the next A-A interval(s), the resultingintrinsic V-V interval is “modified.” The early PVC creates a relativelyshort V-V interval (e.g., V sense-PVC), which cannot be avoided;however, by deliberately and precisely shortening the A-A interval(which can be controlled via the VPP), the next V-V (PVC-V sense)interval will likely be shorter. This avoids the short-long pattern thatis potentially disruptive. By repeating this procedure over a number ofcardiac cycles, the resulting V-V interval is gradually returned to thepre-PVC duration; hence the smoothing effect.

A similar approach is taken with a PVC that occurs after a conductedventricular event in a given A-A interval. Conventional practice was todelay the next cycle (e.g., initiate a VA interval from the PVC). Withthe rate smoothing VPP, the A-A interval is again controlled (andshortened) to effect a desired V-V interval and avoid large relativechanges from one cycle to the next.

The second generalized VPP smoothing variation occurs without a PVC in agiven cycle. Again, a VPP will not provide ventricular pacing in acardiac cycle when a ventricular event occurred in the immediatelyprevious cycle. This provides the greatest chance that intrinsicconduction will occur, even at relatively long P-R intervals that wouldnot be tolerated by a mode such as DDD/R. With that said, for a givenpatient the AV intervals will be reasonably consistent for a given rate,even if long in comparison to other pacing mode standards. Thus, a rangeis established as to when a sensed ventricular event is expected tooccur for a given patient. If no event is sensed during this range, anearly atrial pace is scheduled. In other words, the IMD 10 determinesthat a ventricular event is unlikely in this cycle and seeks to initiatethe next cycle as early as feasible. The reason for this action is thatin the vast majority of situations where a ventricular beat is“skipped”, normal conduction will return in the next cycle. Thus, thisnext cycle is started early. Since the truncated cycle was devoid ofventricular activity, ventricular pacing is now available; however, thePAV (paced AV interval) is set to a value as long as or longer than theexpected intrinsic AV and this promotes intrinsic conduction. Ifconduction fails, the ventricular pace is delivered, generally no laterthan it would have been had the previous A-A interval not beentruncated. The net result is that if ventricular pacing is required, thepreceding V-V interval is no longer than it would be with otherembodiments of the VPP (and would be shorter in some cases).Furthermore, this function very likely results in an intrinsicventricular event following a skipped beat rather than requiring aventricular pace. In addition, the timing of the V-V intervals withintrinsic conduction will vary less relative to one another, despite theskipped beat in the truncated A-A interval.

In various rate smoothing algorithms, the IMD 10 utilizes dataindicative of the patient's intrinsic AV interval (AP-VS) tostrategically pace the atrium to achieve the smoothing. Consideration isalso given to a possible retrograde p-wave after a PVC so as to not pacethe atrium while refractory since the pace will not capture the atrium.

FIG. 10 is a block diagram of the IMD 10 with a number of modulesrepresented that provide for the various embodiments and functionsdescribed herein. The various modules may be used alone or in anycombination. As illustrated, there is a VPP ventricular smoothing module400 having a PVC module 400 a and an expected AV time module 400 b. APAVB modification module 405 is provided that provides at least a longand short PAVB for use when ventricular pacing is or is not provided ina relevant time period. The crosstalk evaluation module 410 determineshow ventricular sensed events occurring in the crosstalk window arehandled and may classify certain events as PVCs. A disablement module415 evaluates various events and is able to either disable (or suspend)the VPP or recommend that action. There is also a PVC discriminationmodule 420 that attempts to identify the presence or absence of a PVCusing the various mechanisms discussed. Finally, there is an atrialpacing evaluation module 425 that evaluates the necessity of atrialpacing, as will be described herein.

One of the benefits of using a VPP is the reduction in ventricularpacing. Not only does this reduce the physiological effects created byventricular pacing it also conserves power in the implantable device.Atrial pacing (or the resulting waveform) more closely approximatesatrial depolarization. This is due to the ability to place the lead inthe right atrial chamber proximate to or at least not in seriouscontravention with the SA node. As a result, there is generally verylittle hesitation in providing atrial pacing. Thus, while there is notnecessarily any known negative physiological effect from atrial pacing,there may be an over reliance upon the function and possibly a tendencyto over utilize the capability.

The atrial pacing evaluation module 425 works in conjunction with theVPP to potentially reduce atrial pacing, particularly rate responsivepacing. That is, the IMD 10 will periodically evaluate the necessity ofatrial pacing. A rate response module takes data from one or morephysiological sensors to determine a pacing rate (or pacing may be setto a particular constant rate). By withholding atrial pacing, theunderlying atrial rhythm or rate will emerge. For those patients who aretruly chronotopically incompetent, atrial pacing will be necessary andwill be provided. However, in other patients the activity sensorindicated rate (or programmed rate, e.g., LRL) is compared with theintrinsic atrial rate. Presumably, atrial pacing would already beinhibited where the sensor rate is below the intrinsic rate; the atrialpacing evaluation module permits reliance upon the intrinsic rate evenif lower than the sensor rate assuming that they are sufficiently close.The particular difference that is tolerated will be programmable by theclinician. For example, as long as the intrinsic rate is within e.g.,10% of the sensor rate, the intrinsic rate is permitted to control. Inother embodiments, the difference may be 5-25%. The variations may be ona beat per minute basis and may be different at different rate levels.Of course, whenever required for any given therapy as described hereinor in other known therapies (e.g., atrial overdrive pacing) the IMD 10will provide atrial pacing as needed.

The present invention has been shown and described with respect tovarious embodiments. One of ordinary skill in the art will recognizethat numerous variations and combinations not specifically described arewithin the spirit and scope of the present invention.

1. A method comprising: operating an implantable medical device (IMD)according to a ventricular pacing protocol (VPP) wherein a sensedventricular event in a first cardiac cycle precludes cardiac pacing in asecond cardiac cycle immediately subsequent to the first cardiac cycle;initiating a first post atrial ventricular blanking (PAVB) intervalfollowing a first atrial pacing pulse under a first condition;accelerating the first cardiac cycle by delivering a second atrialpacing pulse to terminate the first cardiac cycle early in response to afirst sensed indicator; initiating a second PAVB following the firstatrial pacing pulse under a second condition, wherein the first PAVB isshorter in duration than the second PAVB; and providing a crosstalkwindow subsequent to the atrial pacing pulse during which time sensingis permitted on a ventricular channel and events sensed during thecrosstalk window are evaluated according to a feed forward protocol. 2.The method of claim 1, wherein the first condition is when a ventricularpacing pulse has not been provided within a first predetermined intervalprior to the atrial pacing and the second condition is when aventricular pacing pulse has been provided within the firstpredetermined interval prior to the atrial pacing pulse.
 3. The methodof claim 2, wherein the feed forward protocol classifies a current eventsensed during the crosstalk window as a PVC unless a predeterminednumber of immediately preceding cardiac cycles each contained aventricular event sensed during their respective crosstalk windows inwhich case the current event is classified as crosstalk.
 4. The methodof claim 3, wherein a PVC is considered a sensed ventricular event forthe VPP and precludes ventricular pacing in the next cardiac cycle. 5.The method of claim 2, wherein the first sensed indicator is a prematureventricular contraction (PVC).
 6. The method of claim 2, wherein thefirst PAVB is about 8.5 ms and the second PAVB is about 30 ms.
 7. Themethod of claim 2, wherein the first sensed indicator is an expirationof an AV interval timer.
 8. The method of claim 7, further comprising:initiating the second cardiac cycle with delivery of the second atrialpacing pulse so that ventricular pacing is permitted in the secondcardiac cycle; and initiating a paced AV (PAV) interval timer having aduration expected to permit intrinsic conduction to occur and to delivera ventricular pacing pulse upon termination of the PAV if no ventricularevent has been sensed prior to termination of the PAV.
 9. The method ofclaim 5, further comprising: modifying atrial interval timing over aplurality of cardiac cycles to return a heart rate to a pre-accelerationrate.
 10. An implantable medical device (IMD) comprising: a processor; apulse generator in communication with the processor for selectivelyproviding cardiac pacing stimuli; a lead system operably coupled to thepulse generator and the processor to deliver the cardiac pacing stimuliand to sense electrical signals representative of cardiac data; aventricular pacing protocol (VPP) module operably coupled to theprocessor and configured to receive data indicative of a sensedventricular event in a first cardiac cycle and to preclude cardiacpacing in a second cardiac cycle immediately subsequent to the firstcardiac cycle; a pacing control module coupled with the processor andconfigured to initiate a first post atrial ventricular blanking period(PAVB) following an atrial pacing pulse if a first condition is met andinitiate a second PAVB, longer in duration than the first PAVB, if asecond condition is met and to run a crosstalk window during whichsensed ventricular events are classified as a PVC unless a predeterminednumber of immediately preceding cardiac cycles each contained aventricular event sensed during their respective crosstalk windows inwhich case the current event is classified as crosstalk; and a VPP ratesmoothing module configured to deliver a second atrial pacing pulsefollowing the first PAVB to terminate a current cardiac cycle early inresponse to a fist sensed indicator.
 11. The IMD of claim 10, whereinthe first condition is when a ventricular pacing pulse has not beenprovided within a first predetermined interval prior to the atrialpacing and the second condition is when a ventricular pacing pulse hasbeen provided within the first predetermined interval prior to theatrial pacing pulse.
 12. The IMD of claim 11 wherein the first sensedindicator is a premature ventricular contraction (PVC).
 13. The IMD ofclaim 11 wherein the first sensed indicator is an expiration of an AVinterval timer.
 14. The IMD of claim 11 modifying atrial interval timingover a plurality of cardiac cycles to return a heart rate to apre-acceleration rate.
 15. An apparatus comprising: means for operatingan implantable medical device (IMD) according to a ventricular pacingprotocol (VPP) wherein a sensed ventricular event in a first cardiaccycle precludes cardiac pacing in a second cardiac cycle immediatelysubsequent to the first cardiac cycle; means for initiating a first postatrial ventricular blanking (PAVB) interval following an atrial pacingpulse under a first condition; means for initiating a second PAVBfollowing the atrial pacing pulse under a second condition, wherein thefirst PAVB is shorter in duration than the second PAVB; means foraccelerating the first cardiac cycle by delivering a second atrialpacing pulse to terminate the first cardiac cycle early in response to afirst sensed indicator; and means for providing a crosstalk windowsubsequent to the atrial pacing pulse during which time sensing ispermitted on a ventricular channel and events sensed during thecrosstalk window are evaluated according to a feed forward protocol. 16.The apparatus of claim 15, wherein the first condition is when aventricular pacing pulse has not been provided within a firstpredetermined interval prior to the atrial pacing and the secondcondition is when a ventricular pacing pulse has been provided withinthe first predetermined interval prior to the atrial pacing pulse. 17.The apparatus of claim 16, wherein the feed forward protocol classifiesa current event sensed during the crosstalk window as a PVC unless apredetermined number of immediately preceding cardiac cycles eachcontained a ventricular event sensed during their respective crosstalkwindows in which case the current event is classified as crosstalk. 18.The apparatus of claim 17, wherein a PVC is considered a sensedventricular event for the VPP and precludes ventricular pacing in thenext cardiac cycle.
 19. The apparatus of claim 17, wherein thepredetermined number of immediately preceding cardiac cycles is one. 20.The apparatus of claim 17, wherein the first PAVB is about 8.5 ms andthe second PAVB is about 30 ms.
 21. The apparatus of claim 16, whereinthe first sensed indicator is a premature ventricular contraction (PVC).22. The apparatus of claim 16, wherein the first sensed indicator is anexpiration of an AV interval timer.
 23. The apparatus of claim 16,further comprising: means for initiating the second cardiac cycle withdelivery of the second atrial pacing pulse so that ventricular pacing ispermitted in the second cardiac cycle; and means for initiating a pacedAV (PAV) interval timer having a duration expected to permit intrinsicconduction to occur and to deliver a ventricular pacing pulse upontermination of the PAV if no ventricular event has been sensed prior totermination of the PAV.